The invention relates to a process for melt-spinning a side-by-side conjugate filament or yarn with improved control over the denier uniformity and over the shape of the interface between the two polymer components.
In melt spinning a conjugate yarn from a hard polymer and a particular type of polyurethane more fully described below, considerable difficulties were experienced due to variable denier and to a variable shape of the interface between the two polymers. When the yarn is drawn and permitted to relax, a variable bulk level was obtained, attributable to variations in the shape of the interface.
It has been discovered that denier uniformity can be improved and the shape of the interface controlled by heating the polyurethane polymer to a temperature range as defined below prior to its extrusion as part of a conjugate yarn.
Accordingly a primary object of the invention is to provide a process for controlling denier uniformity of a conjugate yarn melt spun from a particular type of polyurethane polymer and from a hard polymer.
A further object is to provide a process for controlling the shape of the interface between the hard polymer and the polyurethane polymer.
Other objects will in part appear hereinafter and will in part be obvious from the following detailed disclosure.
Because minor variations in chemical structure and physical characteristics are difficult to determine adequately in general, the polyurethanes useful according to the invention are most conveniently described in terms of the chemical reactants used to prepare the polyurethane. Broadly, the polyurethanes are made by reacting together (1) a polymeric glycol, which may be a hydroxy-terminated polyester or polyether, having an average molecular weight in the range 800-3000; (2) between 4.6 and 8.8 mols aromatic diisocyanate per mol of polyester or polyether; and (3) sufficient polyol chain-extending agent to provide an NCO/OH ratio between 0.96 and 1.04 to 1.
Suitable polyesters have a molecular weight in the range of about 800-3000 and are obtained by the normal condensation reaction of dicarboxylic acid with a glycol or from a polymerizable lactone. Preferred polyesters are derived from adipic acid, glutaric and sebacic acid which are condensed with a moderate excess of such glycols as ethylene glycol; 1,4-butylene glycol; propylene glycols; diethylene glycol; dipropylene glycol; 2,3-butanediol; 1,3-butanediol; 2,5-hexanediol; 1,3-dihydroxy-2, 2.4-trimethylpentane; mixtures thereof; etc. Useful polyesters may also be prepared by the reaction of caprolactone with an initiator such as glycol, preferably with the molecular weight of the product polyester being restricted to the range 1500-2000. Included among suitable polyethers having molecular weights in the range of 800-3000 are poly(oxyethylene) glycol; polyoxypropylene glycol; poly(1,4-oxybutylene) glycol;
Diisocyanates suitable for the preparation of polyurethanes according to the invention are those diisocyanates wherein the --NCO group is directly attached to an aromatic nucleus, as in 4,4'-diphenylmethane diisocyanate.
Many different common diols or mixtures of diols can be used as the low molecular weight polyol or chain extender. Examples are 1,4 butanediol; ethylene glycol; propylene glycol; and 1,4-B-hydroxyethoxy benzene. The combination of low molecular weight polyol and diisocyanate, as to type and amount, preferably is chosen so as to provide DTA melting point of the polyurethane polymer in the range of 200.degree.-235.degree.C. The polyol should be primarily composed of one or more diols having a molecular weight below 500, although it may be desirable to include as part of the polyol a small molar amount of a multifunctional compound containing three or more hydroxyl groups per molecule. In such a case, the latter compound can have a molecular weight up to 1,500. Amounts up to 0.3 mols of the multifunctional compound per mol of the high molecular weight diol can be used, although ordinarily only about 1/10 or less of this amount (e.g. 0.03 mols or less) need be added for viscosity control. Typical multifunctional compounds are glycerine, trimethylol propane, hexanetriol and the like. When the multifunctional compound is used, the NCO/OH ratio may be between 0.96 and 1.04 to 1; otherwise it should be between 1.01 and 1.04 to 1. The combination of isocyanate and polyol both as to type and amount, must be chosen so as to provide a DTA melting point in the range of about 200.degree.-235.degree.C.
The chemistry and preparation of elastomeric polyurethanes is treated comprehensively in Polyurethanes: Chemistry and Technology, by J. H. Saunders and K. C. Frisch, Part II, Chapter 9, Interscience Publishers, Inc. (1964). U.S. Pat. No. 3,214,411 issued to Saunders and Piggott may be consulted for specific details on the process of preparation of polyester-urethanes for filaments according to the present invention.
Particularly advantageous polyester-urethanes may be made by selecting certain specific reactants and combining them within fairly narrow ranges of proportions as indicated by this general recipe:
100 parts by weight of poly(1,4-butylene) adipate having a molecular weight of 1500-2000;
55-110 parts by weight of 4,4'-diphenylmethane diisocyanate; and sufficient glycol to give a total NCO/OH ratio in the range of 1.01-1.04. The preferred chain-extending glycols are ethylene glycol; 1,4-butane diol; and 1,4-bis-(.beta.-hydroxyethoxy benzene which is the glycol represented by the formula ##SPC1##
In the above formulation the NCO/OH ratio is an abbreviation for the ratio of equivalents of isocyanate groups to the total equivalents of hydroxy groups in the chain-extending glycol combined with the reactive groups in the polyester. The optimum molecular weight and polymer melt strength for maximum spinning speeds without the breaking of fine denier filaments are obtained when the NCO/OH ratio is in the range of about 1.01-1.04.
The polyurethanes in filaments of the invention are regarded as block copolymers in which the polyurethane block melts at a temperature above about 200.degree.C. but below about 235.degree.C. This melting point is measured by differential thermal analysis (DTA), and is indicated by a distinct endothermic peak in the thermogram as the base temperature of the polymer sample is raised. A general description and discussion of DTA methods is given in Organic Analysis, edited by A. Weissberger, Vol. 4, pp. 370-372, Interscience Publishers, Inc. (1960), and in various encyclopedias of Chemical technology. In the examples cited below, the DTA melting points were measured with a commercial duPont 900 DTA Instrument, manufactured by E. I. duPont de Nemours, Inc.
The two components (polyurethane-polyamide) are preferably extruded through single spinneret orifices in side-by-side relation; this arrangement provides the highest order of retractive force to the crimps. However, it is possible to extrude the two components through separate juxtaposed orifices and to coalesce the two extruded streams of molten polymer just below the extrusion face of the spinneret; this method is preferred with higher melting polyamides, such as nylon 66. When a crimp of reduced retractive force can be used a sheath-core structure of the polymers is made, provided that the core is eccentrically arranged with respect to the long axis of the filament. The sheath-core structure is preferred where extremely uniformed dyed appearance in the ultimate textile product is of importance. The two components are preferably present in approximately equal amounts by weight, but the relative amounts of the two components may vary from about 20-80% to 80-20% and a highly crimped structure is assured when at least 30% of the cross section of the spun filament is comprised of the polyurethane component. After extrusion the composite filament must be stretched. The filament can be cold-stretched or, if desirable, be hot-stretched as long as the desired tensile strength is obtained without unduly disrupting the adherence of the two components.